by Bill Robinson
I’ve heard it said that there are two types of windows:
ones that leak and ones that will leak. That may sound a little
harsh, but the fact is that leaks frequently occur at windows.
The perimeter flashings are a good first line of defense, but
if the window itself fails to keep water out — a fairly
common occurrence — the framing and interior finishes can
get wet. The surest way to avoid that problem is to install
sill pans under all windows and exterior doors.
A sill pan is a three-sided pan installed across the bottom of
the opening and integrated into the weather-resistive barrier
(WRB). Any water that makes it through to the rough opening
will be caught by the pan and drained to the surface of the
WRB. While there is a cost to installing a sill pan, it’s
a lot less than the price of a callback or a lawsuit.
On the West Coast, where I live, sill pans have traditionally
been bent and soldered from galvanized sheet metal —
expensive work done at a sheet-metal shop. But in recent years,
installing pans has become cheaper and easier. They can be
purchased as ready-to-assemble kits or fabricated on site from
coil stock or peel-and-stick flashings (see “Sources of
Supply”). In this story I’ll explain how to make
and install all three types.
Bending Pans on Site
Site-bent pans can be made from vinyl or metal coil stock to
fit any size door or window opening. They’re not
difficult to fabricate, but you do need a brake. I use a Mark
II TrimMaster — the TM460 “contractor model”
— with a 4-foot 6-inch capacity (Van Mark Products,
light, and easy to transport, the tool costs about $1,000. The
slitter attachment for cutting the flashing material costs
another $325 — money well spent, in my opinion.
Coil stock is inexpensive: a 2-foot-by- 50-foot roll (vinyl or
painted aluminum) retails for about $80. So, for example, if
the openings are 3 feet wide, you’ll get 39 pans from a
roll, at a material cost of less than $2 each. I prefer vinyl
to aluminum because building materials like fiber cement and
stucco contain cement, which tends to corrode aluminum. The
front edge of door pans is often visible, so if the budget
allows, I make those pans from copper.
Considering set-up time, it might not make sense to site-bend
pans for just one or two windows or doors. But if you’re
installing a lot of pans, bending them yourself is definitely
the most cost-effective way to go. Bending the stock. There are
two 90-degree bends along the length of the pan — a
1/2-inch upturned dam in back and a 2-inch downturned lip in
front. The flat part in between is the width of the jamb
— 4 9/16 inches for the pans shown here. After cutting
the vinyl coil stock to rough length, I use the slitting
attachment to cut it to the right width — 7 1/16 inches
— then make the bends. Gauge blocks are useful for
quickly positioning the material in the brake (see Figure
Figure 1. After bending a short lip on
the back of the vinyl coil stock, the author uses 49/16-inch
gauge blocks (top) to quickly position the material for a
second bend (middle). Because vinyl springs back, the corners
must be slightly overbent (bottom).
Once these two bends are made, I make the end dams (Figure 2),
using ordinary PVC cement — the kind used to solvent-weld
pipe — to secure the laps. There’s no need to use
primer: I just wipe the surface clean and apply glue to one of
the laps, sliding the mating surfaces against each other to
spread the glue. Then I unfold the corner, refold it, and clamp
the joint for a few minutes until the glue has set. If
I’m making the pan from metal, I glue the joint with
sealant and use two-sided tape to hold the connection until the
Figure 2. The author slits the bent
flashing once down the center and again 49/16 inches in from
the end (top left). This allows him to form a face flange (top
right) and to fold in the end (middle left) to create an end
dam (middle right). He glues the lap with fast-setting PVC
cement (bottom left), clamping the joint until the glue has
cured (bottom right).
To complete the pan, I cut the piece to exact length, then
fold and glue the other end. The length, measured from the
finished end dam, should be the width of the rough opening plus
the width of the jamb.
Installing the pan. The pan can be
installed as soon as the glue is dry. It has to slope to drain
to the exterior, so before installing it, I nail a scrap of
beveled siding — or other material ripped to a bevel
— to the rough sill. Since it’s best not to drive
fasteners through the pan, I hold it in place with construction
I slit the housewrap so that it laps over the flanges on the
front of the pan (Figure 3), lap the housewrap covering the
trimmers onto the end dams, and seal the seams with
peel-and-stick flashing. When installing the window, I squirt a
bead of sealant along the rear dam to prevent wind-blown rain
from getting in.
Figure 3. The pan’s face flanges
tuck under the housewrap (top). As an added measure,
peel-and-stick flashing laps over the flanges and end dam. With
the housewrap secured by staples and tape, the opening is ready
for the window (middle). Note that the pan is installed over
sealant, and that the window unit will be pressed into a single
bead of sealant at the inside edge of the pan (bottom). There
should be no sealant directly under the window at the front of
the pan — or there should be gaps (weeps) in the sealant
bead — so that any water that gets in can drain
Flexible Flashing Pans
Sill pans have long been made by wrapping the bottoms of rough
openings with peel-and-stick membrane material, but getting a
perfectly waterproof seal with this method is difficult because
the membrane has to be slit and folded at inside corners. In
the last few years a number of companies have introduced
flexible flashings — peel-and-stick material that can be
stretched to fit around three-way corners. It’s simple to
make pans from these materials; all that’s required is a
beveled piece of wood and a roll of flexible flashing.
On the job shown here (Figure 4), I’m installing a pan
on a home that will be clad with traditional three-coat stucco.
In this application the pan and window are installed before the
WRB, which is usually a double layer of Grade D building
Because this house will be
finished in stucco, the sill pan goes in before the building
paper, which will tuck under the apron flashing below the
window opening. The author installs the pan by adhering
flexible flashing to a beveled strip on top of the sill (top),
then stretching it around the corners of the opening (middle).
After stapling the pan to the face of the sheathing, he rolls
the entire surface to ensure a good bond (bottom).
I begin the installation by stapling a strip of flashing paper
below the bottom of the opening. This piece will be an apron
under which the lathers will tuck their building paper. The
apron should extend a good 12 inches beyond the ends of the
opening. I create a slope by nailing a beveled piece of wood to
the bottom of the opening.
I cut the flashing 12 inches longer than the width of the
opening so it can run 6 inches up both sides. The release paper
on the peel-and-stick I’m using, DuPont’s FlexWrap,
is pre-slit in two places. After removing the center strip, I
position the flashing in the opening and use a roller to press
it onto the framing. The next step is to remove the outer piece
of release paper and fan the flashing around the corners of the
opening. I use a cap staple to hold the flashing against the
wall and run a J-roller across the surface to ensure a good
No back dam. The opening is now ready for the
installation of a window. The pan does not have a back dam,
which in theory is the greatest weakness of this approach. Most
carpenters install the window on a thick bead of sealant at the
inside edge of the jamb and count on the sealant to prevent
wind-driven rain from getting through.
An alternative method is to run the flashing far enough into
the opening so there is excess material that can be folded up
to form a dam. Since the edge will not stay up on its own,
you’ll need to nail a stop behind it or make sure the
finish carpenters turn the flashing up before they install the
Cost. Flexible flashing material is
relatively expensive — about $2 per lineal foot. At that
price the pan for a 3-foot opening would cost $8.
Made from plastic, manufactured pans come in pieces so they
can be cut and assembled to fit the opening (Figure 5).
Typically, there’s a straight center section that’s
cut to the width of the opening and glued to left and right end
pieces. Once the glue’s dried, you install the pan the
same way you’d install one bent on site.
Figure 5. Manufactured pans are typically
assembled by cutting the bottom piece to length, then gluing on
A number of companies make these pans. The better ones have
presloped bottoms and are channeled to drain water. Jamb widths
are standard; prices vary based on the length of the
Manufactured pans are the simplest way to go —
they’re easy to assemble and require no special tools.
However, they often have to be special-ordered and can add
around $20 to the cost of a window installation. Dow’s
Weathermate two-piece sill and door pans are the exception;
they add around $5 per opening.
Bill Robinson is a contractor in Arroyo
Grande, Calif. He moderates the JLC Online exterior-details
forum and is a speaker at JLC Live.
To watch the author make a vinyl sill pan, go to
click on the JLC Extra tab.